High side transistor drivers are used to drive a high side transistor that is connected to a positive supply and is floating (i.e., that is not ground-referenced). The transistor may be a MOSFET (metal oxide semiconductor field effect transistor) or an IGBT (insulated gate bipolar transistor). High side transistors may require a voltage translation from a level shifting device such as a high voltage level shifter. Floating transistors such as the high side transistors may be difficult to turn ON and OFF quickly.
Referring now to FIG. 1, an exemplary transistor driver circuit 10 is shown. The transistor driver circuit 10 includes a high side transistor 12 and a low side transistor 14. A high side transistor driver 16 is connected to a gate 18 of the high side transistor 12 and selectively turns the high side transistor 12 ON and OFF. A low side transistor driver 20 is connected to a gate 22 of the low side transistor 14 and selectively turns the low side transistor 14 ON and OFF. The high side transistor driver 16 and the low side transistor driver 20 may be integrated in a transistor driver module 24. For example, the drivers 16 and 20 turn the transistors 12 and 14 ON and OFF to vary current and/or voltage to an output node 26 according to input signals 28 and 30.
Referring now to FIG. 2, an exemplary arrangement of the high side transistor 12 and the high side transistor driver 16 is shown in more detail. The input signal 28 is input to the high side transistor driver 16 through a high voltage level shifter 40 and an inverter 42. For example, the high voltage level shifter 40 may include a high voltage transistor. Although only a single high voltage transistor is shown, additional high voltage transistors may be necessary to implement the high voltage level shifter 40. Typically, integration of multiple high voltage transistors with other low voltage transistors (not shown) on the same die may be difficult.
Further, for certain applications (e.g., such as fluorescent ballast drivers), voltage stress of more than 600 volts may be applied to the high voltage transistors. Circuitry of the high side transistor driver 16 needs to be able to float above ground by a similar voltage level. As a result, specialty semiconductor processes, such as thick oxide SOI (silicon-on-insulator) processes, may be used. However, because of the large junction capacitances of high voltage transistors, the resulting circuit is typically quite slow. For example, some ballast drivers that implement the high side transistor driver 16 as described in FIG. 2 may operate at a maximum of 50 kHz.